Molecular sieves are a commercially important class of crystalline materials having distinct crystal structures with ordered pore structures and characteristic X-ray diffraction patterns. Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents. The adsorptive and catalytic properties of each molecular sieve are determined in part by the dimensions of its pores and cavities. Thus, the utility of a particular molecular sieve in a particular application depends at least partly on its crystal structure. Molecular sieves are especially useful in such applications as gas separation and organic conversion processes.
Molecular sieves identified by the International Zeolite Associate (IZA) as having the framework type MFI are known. ZSM-5 is a known crystalline MFI material and is useful as a catalyst in a variety of organic conversion reactions.
Crystalline ZSM-5 and its conventional preparation using tetrapropylammonium cations as a structure directing agent are taught by U.S. Pat. No. 3,702,886 and U.S. Pat. No. Re. 29,948.
In addition to tetrapropylammonium cations, a large number of other organic nitrogen-containing compounds, including certain diquaternary ammonium compounds, have been known to direct the synthesis of ZSM-5. For example, U.S. Pat. No. 4,585,638 discloses that the synthesis of ZSM-5 can be directed by the diquaternary cation (alkyl)3N+(CH2)6N+(alkyl)3, where the alkyl group is propyl or butyl.
For some acid-catalyzed reactions over molecular sieves, it is beneficial to reduce diffusion lengths of the reagent and/or product molecules by employing a molecular sieve with a reduced crystal size. Small crystals also have the benefit of providing high surface area.
Accordingly, there is a continued need for new methods for making molecular sieves of the MFI framework type, particularly small crystal forms of this material.